[docs] Add a link to the npm package in the README (socketio#1195)

README.md

@@ -4,7 +4,7 @@
 [![Build Status](https://secure.travis-ci.org/socketio/socket.io-client.svg?branch=master)](http://travis-ci.org/socketio/socket.io-client)
 [![Dependency Status](https://david-dm.org/socketio/socket.io-client.svg)](https://david-dm.org/socketio/socket.io-client)
 [![devDependency Status](https://david-dm.org/socketio/socket.io-client/dev-status.svg)](https://david-dm.org/socketio/socket.io-client#info=devDependencies)
-![NPM version](https://badge.fury.io/js/socket.io-client.svg)
+[![NPM version](https://badge.fury.io/js/socket.io-client.svg)](https://www.npmjs.com/package/socket.io-client)
 ![Downloads](http://img.shields.io/npm/dm/socket.io-client.svg?style=flat)
 [![](http://slack.socket.io/badge.svg?)](http://slack.socket.io)